As Mitt Romney's then-financial committee chairman and a good friend of the former Massachusetts governor, Nierenberg had no second thoughts about who to vote for.

2016 has been much harder.

But after much reflection, Nierenburg -- who has voted for a Republican in every presidential election but one since 1976 -- decided that this year, he would cast his ballot for Democratic nominee Hillary Clinton.

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"It was not an easy decision for me."

"From a political point of view, it was not an easy decision for me," Nierenberg told Circa during a sit-down interview in Washington, D.C.

But as the grandson of Jewish immigrants to the United States, Nierenberg said he could not support Republican nominee Donald Trump, who has based his campaign partially on harsh rhetoric toward immigration.

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Check out some old photos of Nierenberg's family.

From left to right: Nierenberg's great grandparents, Samuel and Esther Holtzman; The whole Nierenberg clan from the early 1890s; Nierenberg's great great grandfather Adolph; and his great great great grandfather, Rabbi Abraham Levi Mayer.

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Clinton "emotionally mature and centered"

Because of Trump's immigration policies -- and his "divisive and xenophobic" rhetoric -- Nierenberg said he'd break with his party, and vote for Clinton instead in November.

"Hillary Clinton knows her stuff," Nierenberg wrote in an op-ed announcing his support for Clinton earlier this month.

"She is emotionally mature and centered. She respects and enjoys working with people from all backgrounds. She has the diplomatic skills needed to break the gridlock in Washington and lead our country well."

Nierenberg said that he usually votes Republican because he believes in "well-managed, cost-effective" small government. But to him, a candidates' temperament is equally as -- if not more important than -- their ideology.

"Competency and character are tremendously important," he said. "In fact, without wanting to be punny, they may, for me, 'trump' the issues."

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I wonder about the man's stability. I wonder about his aggression. I wonder about his lack of civility and common decency.

Almost immediately after his op-ed went live, he said, his office started receiving "nasty" and "verbally abusive" phone calls from strangers.

What did those strangers say?

"The only printable word would probably be the word 'traitor,'" Nierenberg said.

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'I don't always fit well inside either party'

Nierenberg has mixed feelings about being called a traitor to Republicans.

On the one hand, he's used to it. He hasn't always sided with his party, and in the past has donated to several Democrats. He's also known for sometimes supporting traditionally Democratic institutions, like Planned Parenthood.

"I don't always fit well inside either party," he said. "So I make my decisions largely on my assessment of the human attributes of the candidate."

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'It's no traitor to be true to your conscience'

On the other hand, Nierenberg disagrees that deciding to vote for a Democrat makes him a "traitor" to his party.

"It's no traitor to be true to your conscience, true to your family history, true to your legacy, and the future of your children," he said. "If that is what is a traitor, than many Americans should join me in saying that they're happy to be guilty as charged."

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Positive responses, too Not all of the phone calls Nierenberg received after publicly endorsing Clinton were negative. In fact, he said the majority were just the opposite.

"I also had about 250 phone calls and text messages and emails from people I know, and so the positive feedback was over 95 percent," he said.

"But the negatives were so sharp, and so uncivil, and so obscene, that I don't easily forget about them."

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Nierenberg joins growing list

Since Trump clinched the Republican nomination, several high-profile Republicans have come out in support of Hillary Clinton.

That list includes Rep. Richard Hanna (R-NY); former EPA administrators William D. Ruckelshaus and William K. Reilly; former U.S. Sen. Larry Pressler; former Michigan Gov. William Milliken; former Hewlett Packard CEO Meg Whitman; and former George W. Bush aide Lezlee Westine.